One component shelf stable structural adhesive and sealant systems are of considerable commercial interest because they offer greater user convenience than the conventionally employed two component structural adhesive and sealants, especially if they can be cured at ambient temperatures. In the past a considerable amount of research and development has been carried out to develop such products. Thus one component epoxy compositions have been developed as adhesives but they require extended elevated temperature cures to attain useful strength properties. Urethane and silicone based adhesives have become available which do cure at ambient temperatures but their cure mechanism is dependent on the availability of water, usually from the atmosphere, which results in serious limitations in their efficacy for many applications. Also cyanoacrylate types of adhesives have been introduced into the adhesives markets which cure in a matter of seconds at ambient temperatures, again by reaction with moisture, even though they have very definite processing and application limitations particularly as regards types of substrates, durability and chemical resistance and they are supplied in a form which can cause severe injury to the skin and eyes.
Perhaps the most successful one component ambient temperature curing adhesive systems which are now employed, are so-called anaerobic compositions which comprise catalyzed polymerizable mixtures based on monomeric acrylate ester monomers. They are normally liquids remaining in that state as long as they contain an adequate amount of oxygen, such as atmospheric oxygen, and polymerization or cure will not commence even though they may contain substantial amounts of room temperature polymerization initiators. When the anaerobic compositions are however placed between surfaces that are relatively impervious to oxygen, the stabilizing effects of atmospheric oxygen is removed and cure commences within a relatively short time. The products now furnished commercially would find many more applications if certain of their critical performance parameters could be improved. These include strength properties particularly toughness, since the cured anaerobics are quite brittle; resistance to chemicals; useful service temperature range which is at present quite limited both as regards upper and lower service temperatures; and range of applicability to different substrates which are restricted mainly to certain base metal surfaces such as brass, steel and aluminum. Also cure speeds vary with differnt metals ranging from fast with brass to slow for aluminum and finishes on metals such as anodizing, plating and passivating, will slow or prevent cures. Special primers may have to be used which obviously lessen the all important user convenience factor of an ambient temperature curing one component adhesive system. An additional vexing problem with the currently produced anaerobic adhesive compositions is that they are most effective only for bonding surfaces with close clearances and lack gap holding characteristics. This is due to a considerable extent to their substantial shrinkage upon cure. Larger clearances furthermore require more material which significantly reduces cure speed and thus productivity in assembly bonding operations. Practical cure temperatures are very temperature sensitive, lower temperatures requiring much longer cure time. Cure at 40.degree. F. for example may require weeks. After many years of research these problems have not been satisfactorily resolved.
The one component structural adhesive systems described below retain the advantageous characteristics of the presently manufactured anaerobic adhesive compositions yet eliminate most of their limiting features thus making them prime candidates for general bonding applications and especially for uses where structural strength of the bond is required together with joint durability, toughness, chemical resistance and thermal stability at elevated temperatures. They are based on polymerizable vinyl ester polymers prepared by the vinyl ester termination of certain polymerizable epoxy resins with methacrylic and acrylic acids, and also polyesters, a chemically compatible ethylenically unsaturated organic liquid monomer such as styrene, diallyphthalate, triallylcyanurate etc. and a catalyst/inhibitor system containing a peroxide type polymerization initiator, accelerators, inhibitors as well as, optionally, thixotropic agents, thickeners, dyes, pigments and other additives which will impart specific use properties desired for production bonding operations and especially where a tough high structural strength joint with durability, chemical resistance and heat stability is needed. The superior performance of the anaerobic compositions disclosed herewith is ascribed to the well known and long proven superior physical strength properties including toughness, excellent chemical resistance, high degree of thermal stability and inherent adhesive characteristics of the epoxy polymer moities and those of selected polyester moities as well as the unique physical and chemical nature of the unsaturated liquid monomers referred to above particularly when compared with those of the polymerized acrylate type monomers which are employed in the presently offered anaerobic adhesive compositions such as di-, tri-, tetra and polyethylene glycol dimethacrylates and the corresponding acrylates, trimethylolpropane trimethacrylate and acrylate etc.
A most important improvement embodied by the novel anaerobic compositions disclosed herewith is a very great reduction in shrinkage upon cure of said above described initially liquid novel anaerobic adhesive compositions. The shrinkage upon cure of the polymerizable modified vinyl ester polymers based upon certain epoxy resins and also polyesters disclosed herewith is substantially below 5%, generally in the order of 2-3% or less. This compares typically with 14.1, 14.5, 14.2 and 14.0% for the di, tri, tetra and polyethyleneglycolmethacrylates, 14% for trimethylolpropane trimethacrylate and over 16% for trimethylolpropanetriacrylate etc. employed in currently offered anaerobics. The much decreased shrinkage of these new anaerobic compositions greatly enhances their inherent effectiveness for adhesive bonding, especially where parts with surface irregularities requiring greater clearances have to be adhered. Furthermore they provide for the much sought after but never hitherto realized gap filling property which is so desirable for both adhesive and sealant applications. The decreased shrinkage is of particular value for adhering parts with variable clearances as it will assure attainment of uniform bond strength with minimal effect of surface irregularities. It is also most advantageous for applications where bonded parts have to undergo repeated thermal cycling etc. which greatly accentuates the dimensional stability requirements of an adhesive.
The completely cured adhesive compositions can be considered to be toxicologically inert and bonded parts present therefore no health problems from handling. Vinyl ester resins of somewhat similar chemical compositions and heat cured with a peroxide initiator have been found to comply with FDA regulation 121.2572 which covers materials intended for repeated use in contact with foods. The liquid compositions and associated materials may be handled safely by personnel following normal industrial standards for good housekeeping and personal hygiene. Those familiar with handling unsaturated polyester and/or epoxy resins will find the precautions necessary for handling the liquid uncured anaerobic compositions disclosed herewith, in bulk the same. They are not regarded as particularly toxic from ingestion. Because of the possible presence of monomers such as styrene, the usual care should be taken to avoid inhalation of vapors and avoid skin and eye contact when handling them in bulk. For adhesive users the only precautions to be taken are those required by good personal hygiene practices.
Anaerobic compositions as outlined above which can provide the following improved performance properties for adhesive and sealant practice would be extremely novel and useful compositions of matter: improved toughness and other physical strength properties which would make the bonded or sealed parts more resistant to vibration and/or impact loading; better resistance to solvents ranging from hot water to organic fluids such as chlorinated hydrocarbons and fuels; improved resistance to thermal degradation as this would permit usage on parts which can be expected to be exposed to increased temperatures in service; resistance to shrinkage upon cure as this would minimize the tendency of the curing sealant or adhesive to pull away from the parts which are to be sealed or bonded or possibly change the dimensions or configuration of the bonded or sealed joint; and freedom from variation of properties of the cured compositions upon aging.